Headlights Part 2: Low/High Beams

Story & Photos By Jim Clark (The Hot Rod MD)

Modern Headlamps are required to produce both a low and high beam. This can be accomplished by using an individual lamp for each function or by a single multi-function lamp. Individual lamps must be positioned as close to the outside edge of the vehicle, in pairs, at the front of the vehicle with the low beam lamp on the outside when mounted parallel or above the high beam lamp when mounted vertically.

Single multi-function lamps must be mounted parallel at the front of the vehicle, as far out to the edge as possible. The height requirement in FMVSS 108 states that the headlamps must be mounted between 22 to 54-inches from the roadway with the vehicle sitting at ride-height, measuring from the centerline of the headlamp.

Separate low and high beam functions are required because high beams cast most of their light straight ahead, maximizing seeing distance but produce too much glare when other vehicles are on the highway. Low beams have stricter control of upward light, and direct most of their light downward and either rightward (in right-traffic countries) or leftward (in left-traffic countries), to provide safe forward visibility without excessive glare or backdazzle.

Lower Beam: FMVSS 108 definition
A beam intended to illuminate the road and its environs ahead of the vehicle when meeting or closely following another vehicle.

Low beam headlamps must project an asymmetrical pattern that provides adequate forward and lateral illumination but controls glare by limiting light directed towards preceding or oncoming drivers. International ECE Regulations require a beam with a sharp, asymmetric cutoff creating a defined separation at the top of the pattern compared to the North American SAE beam standard that allows a more fuzzy transition at the cutoff in the asymmetrical pattern.

The illustration shows the right-traffic, asymmetrical low beam pattern required by both the International ECE Regulations and North American SAE regulations. The photo shows the sharp cutoff on the left (in right-traffic countries) required by the ECE Regulation.

Upper Beam: FMVSS 108 definition
A beam intended primarily for distance illumination and for use when not meeting or closely following other vehicles.

High beam headlamps allow center-weighted light distribution without any control over light directed toward any other highway users. They are only suitable for use when no preceding or oncoming vehicles are present.

The illustration shows the unrestricted symmetrical high beam illumination pattern allowed by both the ECE and SAE standards. The photo shows that there is no cutoff in this unrestricted symmetrical high beam illumination pattern.

Compatibility With Traffic Directionality
Most low-beam headlamps are specifically designed for use on vehicles that drive on only one side of the road. In North America low-beam headlamps have to dip to the right distributing the light beam with a downward/rightward bias to show the driver the road and signs ahead without blinding oncoming traffic. In Europe where you can encounter the situation of driving from a right-traffic country to a left-traffic country it is a legal requirement to adjust the headlamps temporarily so that their wrong-side beam distribution does not blind oncoming traffic. Various means of accomplishing this are specified in each jurisdiction so anyone that might encounter this situation should check to see what the local laws demand.

Creating a High and Low Beam
There are basically three ways to create high and low beam light patterns. They all use a reflector and some use lens optics in combination with the reflector.

Lens Optics
A light source is positioned at or near the focus of the reflector (parabolic or of non-parabolic complex shape). Then a headlamp lens with fresnel and prism optics molded-in, is placed in front of the reflector, refracting (shifting) parts of the light vertically and laterally to produce the required pattern of light distribution.

This is a side view of typical lens optics. Light is dispersed vertically (shown) and horizontally (not shown).

Reflector Optics
Starting in the 1980s, headlamp reflectors began to evolve beyond the simple stamped steel parabolic shape with the introduction of CAD technology that made possible the design of complex-shaped nonparabolic reflectors. These complex-shaped nonparabolic reflectors allow for the distribution of light for high and low beam without the need for lens optics. This is accomplished by creating reflectors with individual segments of specifically calculated, complex contours. Design and production of these requires that extremely tight tolerances must be maintained.

This is a side view of typical reflector optics. It shows how the reflector without the use of lens optics directs the light beam. Note how the low beam filament is placed in the center of the reflector and the high beam filament is offset from center.

Projector (polyellipsoidal) Lamps
Most of the new vehicles use this type of headlamp. They are generally smaller in diameter than those used on early hot rods and don’t fit into the traditional style housings. The system places a filament at one focus point of an ellipsoidal reflector and has a condenser lens at the front of the headlamp. A shield is used to provide for the cutoff needed to control the low beam projection. Some headlamps use an adjustable shield that is articulated to move when changing from high to low beam. Other versions of this headlamp use a fixed shield so a separate headlamp without a shield must be used to provide high beams.

This is a side view of projector optics. A single filament bulb is used and placing or removing a shield from the light path makes the switch between high and low beam. The condenser lens may have slight fresnel rings or other surface treatments to reduce cutoff sharpness.

Mechanically Aimable Headlamp: FMVSS 108 definition
A headlamp having three pads on the lens, forming an aiming plane used for laboratory photometric testing and for adjusting and inspecting the aim of the headlamp when installed on the vehicle.

Visually/Optically Aimable Headlamp: FMVSS 108 definition
A headlamp which is designed to be visually/optically aimable in accordance with the requirements of paragraph S10.18.9 of FMVSS No. 108.

Sealed beam headlamp has three pads raised on the face of the lens for use when aligning and the indexing of the unit in a housing. Headlight is an early-style 10-inch diameter 1932 Ford unit without pads on the face of the lens. Shown in the photo is a typical compact optical headlamp-aiming device.

Headlamp Aiming
Headlamps need to be aimed using equipment designed for that purpose. Automobile dealers and specialty shops may have this type of equipment. It is expensive and requires that the operators have the necessary training to properly aim headlamps. So this is not a do-it-yourself project.

However, when building a vehicle or repairing/replacing headlights it is possible to perform a headlight adjustment that will be close enough for the purposes of taking the vehicle for a professional headlamp alignment.

Aiming Procedure
There are several methods for aligning headlamps.
(1) The simplest one requires that the vehicle be placed on a level surface at 25 feet from a blank wall.
(2) Masking tape is positioned vertically on the wall in reference to the vehicle centerline and the centerlines of both headlamps.
(3) Masking tape is used to create a horizontal reference line to the headlamps.
(4) Adjustment should be made with about a half-tank of gas, the vehicle sitting level and no unusual loading of the vehicle.
(5) Starting with low beam adjustment, position the high intensity zone so it is two-inches below the horizontal line and two-inches to the side of the vertical headlamp line, away from oncoming traffic. Rotate the headlight or turn the adjustment screws to accomplish this aiming procedure.
(6) With high beams on, the high intensity zone should be vertically centered with the exact center just below the horizontal line.
Note: With dual-filament headlamps it may not be possible to aim both the high and low beams using this method, so aiming the low beams correctly takes priority.

This illustration shows the procedure for a simple adjustment of headlights on a vehicle. A more precise adjustment can then be made at a dealer with their precision headlamp aiming equipment.

Headlamp Leveling Systems
Internationalized ECE Regulation 48 that is in force in most jurisdictions outside of North America specifies a limited range within which the vertical aim of the headlamps must be maintained under various vehicle load conditions. It specifies the type of headlamp aiming device that a vehicle must be equipped with under certain criteria to prevent excessive glare directed towards preceding or oncoming vehicles. North American regulations do not require any type of devices like these.

Summary
All vehicles must be equipped with headlamps that provide for both high beam and low beam. They can be separate lamps, one for high beam and one for low beam, or a single lamp with dual filaments. They must be mounted on the front of the vehicle between 22 and 54-inches above the roadway, in pairs located out to the outside edge of the vehicle.

International ECE Regulations and North American SAE Regulations both specify a similar light pattern for both high and low beam. ECE Regulations specify a sharper cutoff for the low beam pattern creating a defined line at the top of the light beam while the SAE Regulations allow for a less defined transition at the top of the pattern. This accounts for the different ways that each achieves the desired results. The different methods of achieving the production of the correct light pattern were shown, including illustrations.

The correct pattern can only be achieved when the headlamps are properly aimed. This needs to be done with professional headlamp aiming equipment, usually only available at a car dealer or specialty shop. However, an individual can do a rough alignment that will be close enough to serve until the vehicle can be taken to a professional for an alignment. Different loading of the vehicle can alter the alignment so ECE Regulations require that vehicles be equipped with automatic aiming devices to correct any misalignment. SAE Regulations have no requirements with regard to this. Headlights Part-3 will cover the selection and installation of headlights on hot rods.